Q: What is the difference between a school teacher and a train?

Q: What is the difference between a school teacher and a train?
A: The teacher says spit your gum out and the train says "chew chew chew".

This is a riddle. There are two reasons why it is funny: the first is that there are so many differences between a teacher and a train, so the person hearing the joke is hardpressed to select only one of them as "the difference". The setup almost amounts to an absurd punchline-less joke as it is.

...in fact, without the lame pun ending, the joke might be better.

The second reason why this is funny is that the train normally (in American English) says "Choo Choo", which is pronounced exactly the same as "Chew Chew", but the former is the onomatopoeia for the noise of a train, and the latter is a verb in the imperative telling the student to continue to chew on the gum.

Better endings for this joke would include:

saying "you can't come up with a single difference?" in an incredulous manner, which would make this an insult

and

saying "one is a person that educates other people, and the other is a machine that moves people around." which would make this into anti-humor.